chore: seal Design Vision v1 and monorepo scaffold

Archive the differentiated YuXinGu product docs, AI engineering system,
design contract, and Go/Vue scaffold. Next execution prioritizes Cece-parity
over early innovation (see .ai/product/STRATEGY.md).

Co-authored-by: Cursor <cursoragent@cursor.com>

.ai/architecture.md

@@ -0,0 +1,51 @@
+# Architecture
+
+## Allowed call graph
+
+```
+UI (user-h5 / mini-program)
+  → packages/sdk
+  → API Handler
+  → Service
+  → Repository
+  → Database
+```
+
+## Forbidden
+
+- Handler → Database (skip Service/Repository)
+- UI → Database
+- UI direct `fetch` to raw URLs (must use `@yuxingu/sdk` or `src/api` thin wrapper)
+- Service importing Handler
+- Circular package imports
+- Giant `utils` / `common` dump packages
+
+## Monorepo map
+
+| Path | Role |
+|---|---|
+| `apps/api` | Only backend |
+| `apps/user-h5` | Primary client (Vue3+TS) |
+| `apps/mini-program` | Scaffold only until tasked |
+| `apps/admin-h5` | Deferred |
+| `packages/sdk` | Multi-platform HTTP client |
+| `packages/types` | Shared TS types |
+| `packages/utils` | Pure helpers |
+| `packages/ui` | Design tokens |
+| Root HTML / `server.py` | Legacy — do not extend |
+
+## Feature flow (mandatory when adding a feature)
+
+```
+API route + handler
+  → service
+  → repository (if persistence)
+  → migration (if schema change)
+  → SDK / types update
+  → UI page or component
+  → test for money/scoring/auth paths
+  → OpenAPI touch if public API changed
+  → self-review
+```
+
+Missing migration or skipping service layer = incomplete.